Fenton oxidation is an advanced oxidation process (AOP) that is used to oxidize contaminants such as Trichloroethylene (TCE) and Tetrachloroethylene (Perchloroethylene, PCE) in wastewater. The chemical reagent used in Fenton oxidation is known as the Fenton reagent—a solution of hydrogen peroxide and ferrous iron catalyst that was developed in the 1890s by HJH Fenton [1, 2].
In the Fenton reaction, catalyst in the form of transition metals such as iron must be present to initiate the reaction. Iron(II) is oxidized by hydrogen peroxide to iron(III), forming a hydroxyl radical and a hydroxide ion in the process:Fe2++H2O2→Fe3++HO.+OH−  (1)
The free radical generated by this process is a powerful, non-selective oxidant. Oxidation of an organic compound by Fenton's reagent is rapid, exothermic and results in the oxidation of contaminants to carbon dioxide and water primarily [1, 2].
In the conventional Fenton oxidation process, ferrous ions are continuously added to sustain the reaction. Consequently, ferric ions become in excess and are removed from the system via precipitation and sedimentation after the pH is adjusted to between 8-9. The conventional Fenton oxidation system 100 as shown in FIG. 1 comprises a 1st pH adjustment tank 110 for adjusting the pH value of the wastewater to be treated by adding acidic reagent and dosing the wastewater with ferrous reagents, a Fenton reaction tank 120 for adding H2O2 to ferrous reagent-dosed wastewater and allowing Fenton reaction; an air purging tank 130 for purging the unreacted H2O2 and air bubbles generated during Fenton reaction, a 2nd pH adjustment tank 140 for receiving caustic reagent to adjust the pH value of the air-purged wastewater, a coagulant dosage tank 150 for receiving coagulant reagents to form solids, and a sedimentation tank 160 for precipitating the solids.
In practical industrial applications, gravity-based solid-liquid separation has the following shortcomings:
1. Iron complexes and AOP by-products usually exists as tiny pin-flocs, which does not settle easily. Additional chemicals such as polyacrylamide (PAM) or other flocculants must be added to facilitate the gravitational separation.
2. Gas bubbles generated during Fenton reaction can attach to the flocs to cause buoyancy that hinders sedimentation. As a solution, a separate air purging tank is required.
3. Poor effluent qualities due to limitation of sedimentation
Microfiltration and Ultrafiltration processes are excellent alternatives to gravitational sedimentation because high pH ranges allow for iron complexes to exist in insoluble forms, which can be easily retained by the membranes to produce effluent qualities better than gravitational sedimentation. This is especially so when an active layer of rejected iron complexes have been formed on the membrane surface to provide enhanced iron complex rejection via charge repulsion [3, 4]. However, this active layer, which is also a membrane fouling layer, can render the process unsustainable if suction pressures are left unchecked. Thus, it is imperative to operate the Microfiltration or Ultrafiltration with appropriate preventive measures (via Maintenance Cleaning) to mitigate fouling.
The coupling of membrane filtration to Fenton Oxidation has been previously studied and reported to be a promising enhancement of the conventional counterpart. However, studies have been rather limited and mostly restricted to highly controlled laboratory-scale experiments treating model pollutants [5, 6]. The more relevant pilot-scale studies either utilized membrane filtration as a pre-treatment for downstream Fenton processes [7, 8], or en on the side of running non-representative experimental durations and an inability to demonstrate stable membrane performances [9].